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|Plants evolved from green algae in an aquatic evironment about 500 million years ago (see Figure 10-31). There are some 7000 species of green algae living today. They include microscopic, unicellular forms like chlorella and chlamydomonas; colonial forms like the filamentous spirogyra; and multicellular forms like ulva, the sea lettuce. Although some of the multicellular forms are large, they never develop more than a few differentiated types of cells and their fertilized eggs do not develop into an embryo.|
Figure 10-31 Evolution of Plants [view large image]
Figure 10-32 Plant Reproduction
About 400 million years ago, plants began to establish themselves on land. The animals soon followed. The following table compares the aquatic and land environments and the kind of adjustments the organisms had to make in order to live on land.
|The surrounding water prevents the organism from drying out; i.e., it prevents desiccation.||To prevent desiccation, the organism obtains water, provides it to all body parts, and possesses a covering that prevents evaporation.|
|The surrounding water buoys up the organism and keeps it afloat.||An internal structure helps a large body to oppose the pull of gravity.|
|The water prevents desiccation and allows easy transport of reporductive units, such as zoospores and swimming sperm.||In plants, the repoductive units may be adapted to transport by wind currents or by motile animals. Aninals may provide a water environment for swimming sperm.|
|The surrounding water prevents the fertilized egg (zygote) from drying out.||The developing zygote is protected from possible desiccation.|
|The water maintains a relatively constant environment in regard to temperature, pressure, and moisture.||The organism may be capable of withstanding extreme external fluctuations in temperature, humidity, and wind.|
Land plants first developed as mosses living in moist places. These are fairly simple plants that do produce a number of differentiated cell types and whose fertilized egg grows into a distinct embryo. However, they don't have vascular tissue9 (xylem and phloem) to transport water and minerals from their roots up to their leaves. Ferns and horsetails are primitive vascular plants. They still require a wet environment for fertilization to take place. Swamps in the Carboniferous era are believed to have contained fernlike foliage in great abundance.
As suggested in Table 10-04, land dwelling organisms should be able to support themselves against gravity and to protect their reproductive units from desiccation. These requirements are fulfilled by the vascular tissues and embryo sporophytes in gymnosperms (naked seed) and angiosperms (capsuled seed). Figure 10-32 shows the reproductive cycle of the flowering plants. The first stage of sexual reproduction is pollination. This is the transfer of pollen from the stamen to the stigma. When pollen is carried from the stamen to the stigma of the same plant, the process is called self-pollination. When pollen is carried to the stigma of another plant, it is called cross-collination. Most plants have developed ways of avoiding self-pollination because it reduces genetic variation. When a grain of pollen lands on a stigma, a tube, called a pollen tube, grows from the sigma down into the overy. The male gametes in the pollen pass down this tube and meet the ovule. The fusion results in the formation of a seed which contains a plant embryo. The ovary ripens to form the friut around the seed. In order to prevent overcrowding and competition for space, light, and water, seeds and fruits are carried away from the parent plant by wind, animal or explosion. Further details of plants living through the geological periods from Cambrain to the present are described in "Evolution of Micro-organisms and Plants". More details about the living plants can be found in "Anatomy of Plants".
9Vascular tissue is made up of two parts: xylem and phloem. Xylem carries water and minerals up the plant. Phloem carries dissolved foods such as glucose around the plant. Transpiration is the evaporation of water out of the plant, through pores in the leaves called stomato. As transpiration takes place, water is forced up the stem xylem and into the leaves, to replace the water that is lost.